Reproductive success vs fitness: Testing the genetic correlation between number of children and grandchildren in a range of preindustrial societies

Evolutionary fitness is usually measured by number of offspring, but this measure may not reflect the genetic contribution to subsequent generations (i.e. true fitness) if offspring vary in their own ability to survive and... [ view full abstract ]

Evolutionary fitness is usually measured by number of offspring, but this measure may not reflect the genetic contribution to subsequent generations (i.e. true fitness) if offspring vary in their own ability to survive and reproduce. Trade-offs between offspring quantity and quality are especially plausible in humans, given the intense and extended care children require to survive to reproductive age. Using number of children as a proxy for fitness may therefore yield incorrect inferences about selection pressures and projections of evolutionary change. A previous paper (Zietsch et al., 2014, PNAS) showed that the same genes influence both number of children and grandchildren in industrialised Sweden, indicating that number of children does adequately measure fitness in this sample. However, the conclusions from this study may not apply to traditional, natural-fertility societies with higher birth and mortality rates and often scarce or unreliable resources. To test this, we investigated the genetic variance of and covariance between the number of offspring and grandoffspring of individuals from 46 small-scale traditional societies with a total N >45,000. The findings, to be discussed, have important implications regarding our understanding of the process of evolution in humans.